The present invention relates in general to substrate manufacturing technologies and in particular to methods for methods for optimizing the delivery of a set of gases in a plasma processing system.
In the processing of a substrate, e.g., a semiconductor wafer or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate (chemical vapor deposition, plasma enhanced chemical vapor deposition, physical vapor deposition, etc.) for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon.
In an exemplary plasma process, a substrate is coated with a thin film of hardened emulsion (i.e., such as a photoresist mask) prior to etching. Areas of the hardened emulsion are then selectively removed, causing parts of the underlying layer to become exposed. The substrate is then placed in a plasma processing chamber on a substrate support structure comprising a mono-polar or bi-polar electrode, called a chuck. Appropriate etchant source gases (e.g., C4F8, C4F6, CHF3, CH2F2, CF4, CH3F, C2F4, N2, O2, HBr, Ar, Xe, He, H2, NH3, SF6, BCl3, Cl2, etc.) are then flowed into the chamber and ionized to form a plasma to etch exposed areas of the substrate.
A reliable supply of ultrapure electronic specialty gases is critical to maintaining plasma processing system uptimes necessary to maximize productivity and manufacturing yield in semiconductor fabs. The delivery of such gases poses special challenges, however, because of their highly corrosive and reactive nature.
In particular, corrosion created in the gas delivery system may substantially reduce substrate yield. For example, in the process of etching a substrate, pollutants may be generated from materials in the etchant gases (e.g., carbon, fluorine, hydrogen, nitrogen, oxygen, silicon, boron, chlorine, etc.), from materials in the substrate (e.g. photoresist, silicon, oxygen, nitrogen, aluminum, titanium, etc.), or from structural materials within the plasma processing chamber or gas delivery system (e.g., stainless steel, aluminum, quartz, etc.).
It is commonly understood in the art, that a particle one-tenth the size of the thinnest line can substantially damage a substrate. Subsequently, components in contact with the process gases are generally engineered to minimize or eliminate potential sources of system contamination. Since a gas delivery system may be a significant source of contamination, gas conduits are often made of a set metals (e.g., electropolished stainless steel, copper (Cu), aluminum (Al), metal alloys, etc.).
For example, stainless steel is a substantially non-porous material commonly made of iron (Fe), with significant alloying additions of chromium (Cr), which gives the metal its “stainless” or corrosion-resistant characteristics, and nickel (Ni), which stabilizes the austenite and makes the metal nonmagnetic and tough. Electropolishing generally improves the surface chemistry of the part, enhancing the passive oxide film and removing any free iron from the surface.
In general, stainless steel comprises a “passive” film coating which is resistant to further “oxidation” or rusting. This film forms rapidly when exposed to oxygen. Once formed the metal has become “passivated” and the oxidation or “rusting” rate will substantially slow down.
However, plasma processing gasses (e.g., fluorine, chlorine, bromine, etc.) may still eventually penetrate this passive film and will allow corrosive attack to occur, particularly at specific points within a gas delivery system, such as weld bead and heat-affected zones (HAZ). In particular, the process of welding stainless steel often creates slag and layer re-deposits at the weld joints, potentially allowing corrosion. For example, materials such as sulfur (S), manganese (Mn), silicon (Si), and aluminum (Al) may be present at the weld site and tend to react with corrosive plasma processing gases such as the halogen, and produce corrosion and contaminants.
The degree of corrosion and hence the amount of contamination may depend on many factors, such as gas concentration and purity, moisture content, temperature, localized inhomogeneities in material, system flow rates, time of exposure, frequency of exposure. For instance, halogen gases, such as hydrogen chloride or hydrogen bromide, will corrode stainless steel when moisture levels exceed a few parts per million (ppm).
Although moisture can be reduced, it generally cannot be completely eliminated. For example, although plasma processing gases are normally stored in a purified form in compressed gas cylinders, moisture can be introduced into the gas distribution system when the cylinders are replaced, or when maintenance is performed on the processing chamber.
One method for reducing corrosion in the gas delivery system is to coat the inner surface of the tubular structures with a non-corroding material. In one example, a plastic coating is placed on a metal surface as a protective layer through a spraying or dipping process. However, spraying or dipping methods are not readily performed on small diameter delivery lines since access to the inside of the line is limited. In addition, both methods are unable to control the coating uniformity and surface finish, potentially affecting the gas distribution characteristics and substrate yield.
Referring now to FIG. 1 a simplified cross-sectional view of a plasma processing system 100 is shown. Generally, an appropriate set of gases is flowed into chamber 102 through an inlet 108 from gas distribution system 122. These etchant gases may be subsequently ionized to form a plasma 110, in order to process (e.g., etch or deposition) exposed areas of substrate 114, such as a semiconductor wafer or a glass pane, positioned on an electrostatic chuck 116. Gas distribution plate 120, along with liner 112, help to optimally focus plasma 110 onto substrate 114.
Gas distribution system 122 is commonly comprised of compressed gas cylinders 124a–f containing plasma processing gases (e.g., C4F8, C4F6, CHF3, CH2F3, CF4, HBr, CH3F, C2F4, N2, O2, Ar, Xe, He, H2, NH3, SF6, BCl3, Cl2, WF6, etc.). Gas cylinders 124a–f may be further protected by an enclosure 128 that provides local exhaust ventilation. Mass flow controller 126 is commonly a self-contained device (consisting of a transducer, control valve, and control and signal-processing electronics) commonly used in the semiconductor industry to measure and regulate the mass flow of gas to the plasma processing system.
In general, plasma processing gases are often stored compressed gas cylinders 124a–f or several months prior to use in a plasma processing system. Furthermore, many of the plasma processing gases are stored under pressure in a liquefied form. In particular, highly reactive, liquefied plasma processing gas is often typically stored in nickel cylinders to preclude the dissolution of metallic contaminants.
As with the plasma processing system as a whole, moisture in the gas distribution system may react with plasma processing gases to make other contaminant species. For example, HF and mixed tungsten oxyfluorides may be observed upon introducing WF6 into a wet environment.
Referring now to FIG. 2, a simplified diagram of a gas conduit as used in a gas distribution system is shown. For example, a mass flow controller pumps the mixed plasma processing gasses along longitudinal cavity 206 of tube 204 to the plasma processing chamber. Tubes used in gas distribution systems are commonly made of stainless steel in order to resist pitting, corrosion, cracking, and corrosion fatigue.
Referring now to FIG. 3, a simplified diagram of the tubular structure of FIG. 2 is shown, in which corrosion has occurred. As previously described, plasma processing gasses may still eventually penetrate this passive film and will allow corrosive attack to occur, particularly at specific points within the gas delivery system, such as weld bead and heat-affected zones (HAZ), such as on inner surface 304.
In view of the foregoing, there are desired methods and apparatus for optimizing the delivery of a set of gases in a plasma processing system.